The present invention relates generally to completion operations within subterranean wells and, in a preferred embodiment thereof, more particularly provides apparatus and methods for stimulating a subterranean well.
Stimulation operations in subterranean wells are typically performed in portions of the wells which have been lined with protective casing. In general, the casing within a portion of a well to be stimulated is cemented in place so that fluids are prevented from flowing longitudinally between the casing and the surrounding earth. The cement, thus, permits each portion of the well to be isolated from other portions of the well intersected by the casing.
As used herein, the terms "stimulate", "stimulation", etc. are used in relation to operations wherein it is desired to inject, or otherwise introduce, fluids into a formation or formations intersected by a wellbore of a subterranean well. Typically, the purpose of such stimulation operations is to increase a production rate and/or capacity of hydrocarbons from the formation or formations. Frequently, stimulation operations include a procedure known as "fracturing" wherein fluid is injected into a formation under relatively high pressure in order to fracture the formation, thus making it easier for hydrocarbons within the formation to flow toward the wellbore. Other stimulation operations include acidizing, acid-fracing, etc.
Where the wellbore is lined with casing and cement as described above, the stimulation fluids may be conveniently injected into a specific desired stimulation location within a formation by forming openings radially through the casing and cement at the stimulation location. These openings are typically formed by perforating the casing utilizing shaped explosive charges or water jet cutting. The stimulation fluids may then be pumped from the earth's surface, through tubing extending into the casing, and outward into the formation through the perforations.
Where there are multiple desired stimulation locations, which is generally the case, sealing devices, such as packers and plugs, are usually employed to permit each location to be separately stimulated. It is typically desirable for each stimulation location within a single formation, or within multiple formations, intersected by a well to be isolated from other stimulation locations, so that the stimulation operation for each location may be tailored specifically for that location (e.g., in terms of stimulation fluid pressure and flow rate into the formation at that location). The casing and cement lining the wellbore, along with the sealing devices, prevent loss of stimulation fluids from each desired stimulation location during the stimulation operation. In this manner, an operator performing the stimulation operation can be assured that all of the stimulation fluids intended to be injected into a formation at a desired location are indeed entering the formation at that location.
However, it is, at times, inconvenient, uneconomical, or otherwise undesirable to line a portion of a wellbore with casing and cement, even though it may be known beforehand that stimulation operations will need to be performed in that portion of the wellbore. Although such situations arise in vertical and inclined portions of wellbores as well, they frequently arise in portions of wellbores which are generally horizontal.
Reasons why a generally horizontal portion of a well may not be lined with casing and cement are many. Included among these is the fact that casing and cementing operations are particularly difficult to perform satisfactorily in a generally horizontal portion of a well. For example, it is difficult to completely fill voids with cement between casing and the surrounding earth in a horizontal well portion. In particular, it is common for the cement to settle in a bottom part of the horizontal well portion, leaving a longitudinally extending void or mostly water-filled gap between the cement and the upper part of the horizontal well portion.
It may be easily seen that a longitudinally extending void or gap between the cement and the earth surrounding the wellbore will provide fluid communication along the length of the wellbore. This fluid communication will make it impractical, or at least very difficult, to perform stimulation operations at a desired location within the horizontal well portion isolated from other locations.
For this reason and others, generally horizontal well portions are many times left uncased. If it is desired to perform stimulation operations in an uncased well portion, expensive and oftentimes unreliable sealing devices, such as inflatable packers, are typically used to isolate each stimulation location. The cost of such sealing devices, and the expense of running, setting, and testing them, which frequently must be done multiple times due to their unreliability, often makes their use prohibitive.
From the foregoing, it can be seen that it would be quite desirable to provide a method of stimulating a subterranean well which does not require lining a portion of the well with casing and cement, and which does not require the use of sealing devices, such as inflatable packers, in an uncased portion of the well, but which permits each desired location within the uncased portion of the well to be isolated from other portions of the well during stimulation operations therein. It is accordingly an object of the present invention to provide such a well stimulation method and associated apparatus.